Carbon nanotubes hold great promises in many areas of technology and fundamental research. Nanotube technology has a wide variety of applications in industry and sciences, including: field emission, conductive plastics, fuel cells, storage media such as hydrogen storage for fuel cells, conductive adhesives, and many advanced materials to name a few.
Single walled nanotubes and multi walled nanotubes (SWNT and MWNT, respectively) have been formed.
SWNTs can be synthesised by an arc-discharge method, using metal-filled graphite rods. U.S. Pat. No. 5,482,601, issued Jan. 9, 1996 to Ohshima et al., describes such a technique. U.S. Pat. No. 6,063,243, issued May 16, 2000, to Zettl et al., also describes a process and apparatus for producing nano-scale tubes and particles. This document discloses a particular electrode for use arc-discharge methodology. Arc-discharge techniques are impractical for large-scale production of nano-scale tubes and particles, because the yield is low and the resulting product has a high impurity content.
A laser ablation method for nanotube production is known to produce SWNTs with a much higher yield and fewer impurities than the arc-discharge method. An article entitled “Conductivity enhancement in single-walled carbon nanotube bundles doped with K and Br” by Lee et al., Nature 388:255-257 (Jul. 17, 1997) reports that bundles of SWNT were prepared by metal-catalysed laser ablation of graphite. U.S. Pat. No. 6,183,714, issued Feb. 6, 2001 to Smalley et al. describes a method of making SWNTs involving laser vaporization of carbon with a Group VIII transition metals, followed by condensation of the vapor. However, these laser ablation methods are not amenable to large-scale production of nanotubes.
A chemical vapour deposition (CVD) method has been disclosed for nanotube synthesis. The CVD method is capable of controlling growth direction on a substrate. There are two types of CVD syntheses of SWNTs, depending on the form of supplied catalyst. One type of synthesis requires that the catalyst be embedded in a porous material or supported on a substrate. The catalyst is placed at a fixed position within a furnace and heated in the presence of a hydrocarbon gas flow. The other type of CVD synthesis involves use of a gas phase for introducing the catalyst, in which both the catalyst and reactant hydrocarbon gas are fed into a furnace. This is followed by a catalytic reaction in a gas phase. In both methods, catalyst nanoparticles are formed through thermal decomposition of organometal compounds such as iron pentacarbonyl and ferrocene.
Currently available techniques for the synthesis of single-walled and multi-walled carbon nanotubes produce only gram quantities per day of crude material at a very high cost of production Only a fraction of this crude material contains nanotubes, and within the nanotubes there is a distribution of diameters and chirality or “helicity”, in the case of SWNT. The diameter of nanotubes synthesized according to these conventional techniques can range from 0.4 nm to several nanometers, and within a given diameter there could be several chiral species of SWNT. Thus, these techniques suffer from the lack of control in purity of material produced (diameter and chirality). The diameter and chirality of SWNT influence their physical and chemical properties.
SWNTs have applications in several emerging technologies including nanoelectronics, drug delivery systems, fuel cells and chemical sensors. MWNTs, have applications in the fields of nanoelectronics and advanced materials such as composites for use in fuel cell technologies.
Two major problems arise in conventional SWNT synthesis: large-scale production and quality control. By quality control, such parameters as chirality selection, or diameter controlled, and diameter distribution of the resulting nanotubes must be considered Although scaling-up production of nanotubes could be accomplished by increasing the size and number of apparatus used, such an approach would not lead to a decrease in production price or in increase in product quality. Economically speaking, scaling up current methods is not a viable option. Thus, there is a need for an efficient method of nanotube synthesis that can be conducted on a large scale.
Formation of carbon nanotubes is catalyzed by mixed or pure transition metal particles. It has been demonstrated that the diameters of the metal particles influence the diameters of the resulting nanotubes. However, current synthesis methods have no or little control on the size of the metal particles. They rely on the synthesis of nanometer size particles from organometallic precursors. There exists no method for the synthesis of carbon nanotubes with a narrow diameter distribution. Therefore, there is a need for a method of nanotube synthesis that allows for control of diameter and chirality (in the case of SWNT).